In recent years, there have been demands for decreases in the weight of automotive bodies as one measure to decrease the amount of CO2 discharged from automobiles in order to protect the global environment. Decreases in weight cannot be allowed to be accompanied by decreases in the strength demanded of automotive bodies. Therefore, increases in the strength of steel sheets for automobiles are being promoted.
There are also increased societal demands for safety of automobiles in collisions. For this reason, the properties demanded of steel sheets for automobiles are not simply a high strength; there is also a desire for improved impact resistance should a collision occur during driving. Namely, there is a desire for high resistance to deformation when deformation takes place at a high strain rate. The development of steel sheets which can satisfy these demands is being studied.
In general it is known that the difference between the static stress and the dynamic stress of a steel sheet (in this invention, this difference being referred to as the static-dynamic difference) is large in steel sheets made of mild steel and decreases as the strength of steel sheets increases. An example of a multi-phase steel sheet having both a high strength and a large static-dynamic difference is a low-alloy TRIP steel sheet.
As a specific example of such a steel sheet, Patent Document 1 discloses a strain induced transformation-type high-strength steel sheet (TRIP steel sheet) having improved dynamic deformation properties which is obtained by pre-straining a steel sheet having a composition comprising, in mass percent, 0.04-0.15% C, one or both of Si and Al in a total of 0.3-3.0%, and a remainder of Fe and unavoidable impurities and having a multi-phase structure comprising a main phase of ferrite and a second phase which includes at least 3 volume percent of austenite. The pre-straining is carried out by one or both of temper rolling and a tension leveling such that the amount of plastic deformation T produced by pre-straining satisfies the following Equation (A). The steel sheet before pre-straining has such a property that the ratio V(10)/V(0) which is the ratio of the volume fraction V(10) of the austenitic phase after deformation at an equivalent strain of 10% to the initial volume fraction V(0) of the austenitic phase is at least 0.3. The steel sheet is characterized in that the difference (σd−σs) between the quasi-static deformation strength as when deformed at a strain rate in the range of 5×10−4-5×10−3 (s−1) and the dynamic deformation strength ad when deformed at a strain rate in the range of 5×102-5×103 (s−1) after pre-straining in accordance with Equation (A) below is at least 60 MPa. Steel sheets having a multi-phase structure are hereinafter referred to collectively as multi-phase steel sheets.0.5[{(V(10)/V(0))/C}−3]+15≥T≥0.5[{(V(10)/V(0))/C}−3]  (A)
As an example of a multi-phase steel sheet having a second phase which is primarily martensite, Patent Document 2 discloses a high-strength steel sheet having an improved balance of strength and ductility and having a static-dynamic difference of at least 170 MPa. The steel sheet comprises fine ferritic grains in which the average grain diameter ds of nanocrystalline grains having a grain diameter of at most 1.2 μm and the average grain diameter dL of microcrystalline grains having a grain diameter exceeding 1.2 μm satisfy dL/ds≥3. In that document, the static-dynamic difference is defined as the difference between the static deformation stress obtained at a strain rate of 0.01 s−1 and the dynamic deformation stress obtained when carrying out a tensile test at a strain rate of 1000 s−1. However, Patent Document 2 does not contain any disclosure concerning the deformation stress in an intermediate strain rate region where the strain rate is greater than 0.01 s−1 and less than 1000 s−1.
Patent Document 3 discloses a steel sheet having a high static-dynamic ratio having a dual-phase structure consisting of martensite having an average grain diameter of at most 3 μm and ferrite having an average grain diameter of at most 5 μm. In that document, the static-dynamic ratio is defined as the ratio of the dynamic yield stress obtained at a strain rate of 103 s−1 to the static yield stress obtained at a strain rate of 10−3 s−1. However, there is no disclosure concerning the static-dynamic difference in a region in which the strain rate is greater than 0.01 s−1 and less than 1000 s−1. In addition, the static yield stress of the steel sheet disclosed in Patent Document 3 is a low value of 31.9 kgf/mm2-34.7 kgf/mm2.
Patent Document 4 discloses a cold-rolled steel sheet having improved impact absorbing properties in which the structure comprises at least 75% of a ferritic phase having an average grain diameter of at most 3.5 μm and a remainder of tempered martensite. The impact absorbing properties of the cold-rolled steel sheet are evaluated by the absorbed energy when a tensile test is carried out at a strain rate of 2000 s−1. However, there is no disclosure in Patent Document 4 concerning the absorbed impact energy in a strain rate region of less than 2000 s−1.